The present invention relates to radio communication systems. More particularly, and not by way of limitation, the present invention is directed to an apparatus and method for transmitting independent data streams from a plurality of radio antennas.
The capacity of a wireless channel can be enhanced greatly by utilizing multiple transmit and multiple receive antennas. This has been demonstrated by open-loop multiple-input-multiple-output (MIMO) schemes, such as the Bell Labs Layered Space-Time (BLAST) architecture. Open-loop MIMO schemes do not require any feedback link from the receiver to the transmitter, as in closed-loop MIMO schemes. However, by exploiting instantaneous channel-state information provided to the transmitter by the receiver as in closed-loop MIMO schemes, the channel capacity is further enhanced. Closed-loop MIMO schemes include Eigenmode-BLAST, which is an optimum closed-loop MIMO scheme, and Per Antenna Rate Control (PARC), which achieves the open-loop MIMO capacity. In PARC, an independently coded and modulated data stream is transmitted from each transmit antenna. The coding rate for each data stream is provided by the receiver via a feedback mechanism.
While optimum closed-loop MIMO schemes require a large amount of instantaneous channel information to be fed back from the receiver to the transmitter, PARC requires much less feedback. However, PARC does not perform well in certain conditions, such as low signal-to-noise ratio (SNR) conditions, systems with correlated antennas, systems with fewer receive antennas than transmit antennas, and channels with Ricean fading. In such conditions, the MIMO channel often cannot support the transmission of as many data streams as the number of transmit antennas (as done in PARC). This limits the capacity of PARC. This problem was partly solved by Selective-PARC (S-PARC), which is an extension of PARC. S-PARC transmits as many data streams as can be supported by the channel by using the same number of transmit antennas as there are data streams. S-PARC performs better than PARC in the above-mentioned conditions. However, since S-PARC may not utilize all transmit antennas, its capacity is significantly lower than optimum closed-loop MIMO.
Thus, what is needed in the art is a closed-loop MIMO scheme that overcomes the deficiencies of conventional systems and methods by utilizing all transmit antennas to transmit an adaptive number of data streams. The present invention provides such a system and method.